1. Field of the Invention
The present invention relates to a method and an apparatus for estimating a non-linear characteristic of an amplifier and, in particular, to a method and an apparatus which can be used in linearizing non-linear power amplifiers as used in transmitters for digital broadcasting, satellite communications or mobile communications.
The present invention can be used for estimating a non-linear characteristic of an amplifier in case of input signals formed using modulation methods which result in a non-constant envelope of the radio frequency carrier signal. Thus, the present invention preferably finds use with respect to transmitters for digital broadcasting, which are fed by multi-carrier signals, such as OFDM signals (OFDM=orthogonal frequency division multiplex), for example. With such signals, the non-linearity of the amplifier causes unwanted frequency portions of the signal at the output of the amplifier. Such frequency portions interfer with adjacent-frequency channels.
In addition, the present invention is applicable to mobile communication systems using CDMA signals (CDMA=code division multiple access), for example. Additionally, the present invention can be advantageously used in satellite earth stations or base stations of mobile telephone systems.
2. Description of Prior Art
U.S. Pat. No. 5,049,832 relates to a system for the linearization of power amplifiers using a look-up table. The look-up table contains complex pre-distortion values to be applied to an input signal of the power amplifier in order to provide a pre-distortion of the input signal which compensates for distortions caused by the power amplifier with respect to magnitude and phase. U.S. Pat. No. 5,049,832 teaches several methods used to appropriately establish the contents of the look-up table. The prior art methods suffer from the disadvantage that errors occuring when measuring the non-linearity affect the pre-distortion coefficients stored in the look-up table such that the linearization gain which can be achieved is reduced. In an extreme case, the linearity of the power amplifier can even deteriorate.
It is the object of the present invention to provide a method and an apparatus for estimating a non-linear characteristic of an amplifier wherein the influence of errors during the measurement of the non-linearity is reduced.
In accordance with a fist aspect of the invention, this object is achieved by a method for estimating a non-linear characteristic of an amplifier, comprising the steps of:
providing an input signal;
determining a reference signal based on the power of the input signal;
amplifying the input signal or a signal derived from the input signal using the amplifier to produce an amplified signal;
determining a measurement signal based on the power of the amplified signal;
associating respective samples of the measurement signal with respective samples of the reference signal;
determining a gain value of the amplifier for each sample from the respective sample of the reference signal and the associated sample of the measurement signal;
dividing an operating range of the amplifier into a plurality of power sections;
for each power section, averaging the gain values associated with samples of the reference signal, which have a power which lies in the respective power section, in order to produce an averaged gain value;
for each power section, determining a measure of the deviation of the associated gain values from the averaged gain value;
rejecting an averaged gain value which has a measure of the deviation which exceeds a predetermined threshold; and
performing an interpolation using averaged gain values which have not been rejected in order to produce an estimation of the non-linear characteristic of the amplifier.
In accordance with a second aspect of the invention, this object is achieved by an apparatus for estimating a non-linear characteristic of an amplifier, comprising:
means for determining a reference signal based on the power of an input signal;
means for determining a measurement signal based on the power of an amplified signal, said amplified signal being said input signal or a signal derived from said input signal amplified by said amplifier;
means for associating respective samples of said measurement signal with respective samples of said reference signal;
means for determining a gain value of said amplifier for each sample from the respective sample of said reference signal and said associated sample of said measurement signal;
means for dividing an operating range of said amplifier into a plurality of power sections;
means for averaging the gain values associated with samples of said reference signal which have a power which lies in a respective power section, in order to produce an averaged gain value for each power section;
means for determining a measure of the deviation of the associated gain values from said averaged gain value for each power section;
means for rejecting an averaged gain value which has a measure of the deviation which exceeds a predetermined threshold; and
means for performing an interpolation using averaged gain values which have not been rejected in order to produce an estimation of said non-linear characteristics of said amplifier.
The present invention is particularly suited for estimating the non-linear characteristic of a power amplifier in a transmitter output stage, wherein a non-linear phase characteristic and a non-linear magnitude characteristic of the power amplifier can be obtained. This is achieved by determining a complex gain value of the power amplifier which has a magnitude and a phase. In such a case, an averaged magnitude and an averaged phase are determined for each power section and a measure of the deviation of the magnitudes from the averaged magnitude and a measure of the deviation of the phase from the averaged phase are determined for each power section. If the measure of the deviation of the magnitudes from the averaged magnitude exceeds a predetermined threshold, the associated averaged magnitude will be rejected. If the measure of the deviation of the phases from the averaged phase exceeds a predetermined threshold, the associated averaged phase is rejected. Finally, an interpolation is performed in order to establish values for the rejected averaged magnitudes and the rejected averaged phases in order to produce an AM/AM characteristic and an AM/PM characteristic of the power amplifier.
Thus, according to the present invention, the reliability of the measured value, that is of the measurement signal, is considered such that the usage of an erroneous measurement in estimating a non-linear characteristic of an amplifier can be avoided.
The method and the apparatus according to the present invention is insensitive to measuring errors occuring in practice when a reference signal and a measurement signal for estimating non-linearities of a power amplifier are determined. Such errors can occur due to noise of analog-to-digital converters and other circuit elements preceding the power amplifier, phase noise caused by the upconverter preceding the power amplifier or caused by the sampling clock of the analog-to-digital converter preceding the power amplifier. In addition, a memory effect of the power amplifier or the preceding upconverter due to the usage of surface acoustic wave filters can cause measuring errors. Finally, a non-optimal estimation of a delay or a possible frequency shift between measurement signal and reference signal may be the reason for measuring errors. Since the present invention reduces the influence of such measuring errors to the estimation of the non-linearity of the amplifier, less expensive elements can be used in the output stage, for example, less expensive upconverters or analog-to-digital converters.